This invention relates to an spread-spectrum signal receiver apparatus and to an interference cancellation apparatus. More particularly, the invention relates to a spread-spectrum signal receiver apparatus for receiving a spread-spectrum signal, which has been spread by a spreading code comprising a combination of a first code decided by a spreading factor and a second code that differs for every user, and demodulating transmit data from the received signal, and to an interference cancellation apparatus for generating a replica of an interference signal from the received signal.
In spread-spectrum communications, W-CDMA (Wideband Code Division Multiple Access), which employs direct-sequence spreading, is one of the third-generation mobile communications systems the standardization of which is being forwarded by the 3GPP.
With CDMA, as shown in FIG. 10, a mobile station, which is a spread-spectrum signal transceiver, has a first modulator la for applying BPSK modulation (see FIG. 11) to control data that includes pilot data, and a first spreader 1b for applying spread-spectrum modulation using a spreading code for the control data. The mobile station further includes an encoding circuit 1c for subjecting the transmit data (user data) to suitable encoding such as convolutional coding, a second modulator 1d for subsequently applying BSPK modulation and a second spreader 1e for spreading the resultant signal using a spreading code for the user data. The mobile station further includes a multiplexer 1f for mapping the control data and user data, which have been spread by the first and second spreaders, as an I-axis component (I-channel component) and Q-axis component (Q-channel component) of an I-Q complex plane, as illustrated in FIG. 11, and multiplexing the resulting signals, and a radio transmitter unit 1g for subjecting the multiplexed signal to frequency conversion and high-frequency amplification and transmitting the resulting signal from an antenna 1h. It should be noted that the I and Q channels are referred to also as data and control channels, respectively. The spreading codes used in the first and second spreaders 1a, 1e are obtained by multiplying a user identification code (long code) and a channel identification code (short code), which is for identifying the data channel or control channel.
An uplink signal from the mobile station to a base station has a frame format shown in FIG. 12. One frame has a duration of 10 ms and is composed of 15 slots S0 to S14. User data is mapped to the I channel (data channel) and control data, which is data other than the user data, is mapped to the Q channel (control channel). Each of the slots S0 to S14 of the data channel that transmits the user data is composed of n bits, where n varies depending upon the transmission rate. The transmission rate will be 7.5 (=5×15/10×10−3) kbps if n=5 holds and 30 kbps if n=20 holds.
Each slot of the control channel that transmits the control data is composed of 10 bits, and the transmission rate is a constant 15 kbps. Each slot transmits a pilot, transmission-power control data TPC, a transport format combination indicator TFCI and feedback information FBI. The pilot is utilized on the receive side for synchronous detection and SIR measurement, the TPC is utilized for control of transmission power, the TFCI transmits the transmission rate of the data and the number of bits per frame, etc., and the FBI is for controlling transmission diversity at the base station. It should be noted that the data transmission rate and the spreading factor have a 1:1 relationship, and that the spreading factor of the data channel is found from the transmission rate.
Thus, there are instances where the transmission rates on the data and control channels differ. In such case the spreading factor [=(symbol period)/(chip period)] on the data channel differs from that on the control channel. For example, (1) if the transmission rate of the data channel is lower than that (15 kbps) of the control channel, then the spreading factor of the data channel will be larger than that of the control channel, and (2) if the transmission rate of the data channel is higher than that (15 kbps) of the control channel, then the spreading factor of the data channel will be smaller than that of the control channel. The larger the spreading factor, the higher the process gain. Accordingly, in a W-CDMA system, transmission power for which the spreading factor is larger is reduced to lower the total transmission power. In other words, with W-CDMA, the control and data channels are subjected to BPSK modulation to effect spread-spectrum modulation at powers that differ from each other, the spread-spectrum modulated signals are mapped on an I-Q complex plane and multiplexed and the multiplexed signal is transmitted.
If, by way of example, the spreading factor of the data channel is larger than that of the control channel, then, as shown in FIG. 13, the apparatus of FIG. 10 is further provided with multipliers 1h, 1i, the multiplier 1h multiplies the BPSK modulation output of the second modulator id of the data channel by βc (βc<1) and the multiplier 1i multiplies the BPSK modulation output of the first modulator 1a of the control channel by 1 (i.e., leaves this output unchanged). The first and second spreaders 1b, 1e thenceforth spread-spectrum modulate the outputs of the multipliers 1i, 1h, respectively, the multiplexer 1f maps the spread-spectrum modulated signals of the respective channels on the I-Q complex plane, as illustrated in FIG. 14A, and multiplexes the resultant signals, and the radio transmitter unit 1g subjects the multiplexed signal to a frequency conversion and high-frequency amplification and transmits the resulting signal from the antenna 1h. By thus lowering the transmission power of the channel having the larger spreading factor, the total transmission power can be controlled (reduced).
Further, if the spreading factor of the data channel is made smaller than that of the control channel, the multiplier 1h multiplies the BPSK modulation output of the second modulator 1d by 1 and the multiplier 1i multiplies the BPSK modulation output of the first modulator 1a by β. The multiplexer 1f maps the spread-spectrum modulated signals of the respective channels on the I-Q complex plane, as illustrated in FIG. 14B, and multiplexes the resulting signals. As a result, the total transmission power can be reduced by lowering the transmission power of the channel having the larger spreading factor.
FIG. 15 is a block diagram illustrating one channel of the receiver section of a base station. The base station has a radio unit 2a for frequency-converting a high-frequency signal received from an antenna ATN to a baseband signal; a quadrature demodulator 2b for subjecting the baseband signal to quadrature detection, converting the analog in-phase component (I component) and analog quadrature component (Q component) to digital data and distributing the data to a searcher 2c and fingers 2d1˜2dn. Upon receiving input of a direct-sequence signal (DS signal) that has been influenced by the multipath effect, the searcher 2c detects multipath interference by performing an autocorrelation operation using a matched filter and inputs despreading-start timing data and delay-time adjustment data of each path to the fingers 2d1˜2dn. A control-channel despreader 3a of each of the fingers 2d1˜2dn subjects a direct wave or delayed wave that arrives via a prescribed path to despread processing using a code identical with the spreading code for the control channel, integrates the results of despreading, then applies delay processing that conforms to the path and outputs a control-data signal. A data-channel despreader 3b subjects a direct wave or delayed wave that arrives via a prescribed path to despread processing using a code identical with the spreading code for the data channel, integrates the results of despreading, then applies delay processing that conforms to the path and outputs a user-data signal.
A channel estimation unit 3c estimates the fading characteristic of the communication path using the pilot signal contained in the despread control-data signal, executes channel estimation which compensates for the effects of fading, and outputs a channel estimation signal. Channel compensation units 3d, 3e multiply the despread control-data signal and despread user-data signal by the complex-conjugate signal of the channel estimation signal to thereby compensate for fading.
A RAKE combiner 2e combines and outputs the control-data signals output from the fingers 2d1˜2dn. A decoder 2g applies error-correction decoding processing to the data that is output from the RAKE combiner 2e, decodes the control data that prevailed prior to encoding and outputs the decoded data. A RAKE combiner 2i of the data channel combines and outputs the user-data signals output from the fingers 2d1˜2dn, and a decoder 2m applies error-correction decoding processing to the data that is output from the RAKE combiner 2i, decodes the user data that prevailed prior to encoding and outputs the decoded data.
Thus, with the CDMA scheme, a prescribed code is assigned to a user and multiple users communicate simultaneously. However, because signals from other channels currently engaged in calls constitute interference, the number of channels (users) that can communicate simultaneously is limited. Interference suppression techniques such as interference cancellers and adaptive array antennas are effective in increasing channel capacity and research relating to these techniques is progressing.
If we let Tc represent the period (chip period) of a spreading code and let T represent the symbol period of a narrow-band modulated signal that undergoes modulation by the spreading code, then T/Tc will be the spreading factor. By applying spread-spectrum modulation to a narrow-band modulated signal NM, as shown in (A) of FIG. 16, the bandwidth is enlarged by a factor of T/Tc, as indicated by SM, as a result of which the energy is spread. As a consequence, if spread-spectrum modulated signals are emitted from the mobile stations of multiple users simultaneously, the signals overlap one another in the manner shown in (B) of FIG. 16. If a signal from one user, e.g., user 1, is demodulated from these overlapping signals by despreading, the result will be as shown in (C) of FIG. 16. The spread signals of users 2 and 3 constitute interference signals with respect to the narrow-band signal of user 1. The spectrum ratio of the narrow-band signal of user 1 to the interference signal is referred to as the Signal Interference Ratio (SIR). The larger the number of users, the smaller the SIR. This means that there is a limit upon the number of channels that can communicate simultaneously (i.e., that there is a limit upon channel capacity). An interference canceller seeks to enlarge the SIR and thereby increase channel capacity, or to reduce transmission power, by suppressing the spread signals of other users, as depicted in (D) of FIG. 16. Specifically, an interference canceller suppresses interference by generating a replica of an interference signal using the results of demodulating each of the receive signals and subtracting the replica from the receive signal.
FIG. 17 is block diagram illustrating a CDMA receiver of a base station having an interference canceller. Specifically, the receiver includes an interference canceller 101 and receive-signal demodulators 102a to 102k, which are for users 1 to k, respectively, provided for respective ones of receive channels. The interference canceller 101 is provided with interference cancellation units (ICU) 1111˜111k corresponding to respective ones of the receive channels. The interference cancellation units 1111˜111k generate interference replicas of chip rates from the receive signal and output the replicas. More specifically, each of the interference cancellation units 1111˜111k multiplies the receive signal by a dispreading code, then discriminates data using the despread signal, lastly spreads the discriminated data again, thereby generating the interference replica. A combiner 112 combines the interference replica signals of the respective receive channels, a filter 113 limits the band of the combined interference replica signals, a delay unit 114 delays the receive signal for a length of time required for generation of an interference replica, and a subtractor 115 executes interference suppression by subtracting the combined interference replica from the receive signal. The interference cancellation units produce replicates (replicates of control data and replicates of user data) of the transmit signal having the symbol rate. These replicates are referred to as symbol replicas and are transmitted to the receive demodulator after interference is eliminated. As a result, not only is interference from other channels eliminated but so is interference from the multipath effect of the channel in question. The interference cancellation units 1111˜111k are connected in parallel and shorten processing time by processing all channels simultaneously.
FIG. 18 is a diagram showing the structure of each of the interference cancellation units 1111˜111k according to the prior art. Each interference cancellation unit includes a despreader 151 for multiplying the receive signal by a despreading code that is identical with the spreading code, thereby outputting a despread signal; a demodulator 152 for demodulating “1”, “0” of user data and control data on the basis of the result of despreading; an attenuator 153 for attenuating the demodulated signal by multiplying the result of demodulation by a damping coefficient that conforms to the degree of reliability; a re-spreader 154 for spreading the demodulated signal again to thereby output an interference replica; a despread-information extraction unit 155 for identifying the spreading factor on the transmit side by collecting TFCI bits, which are contained in the control data, over the duration of one frame; and a symbol-replica interface 156 for creating and sending a symbol replica.
The despreader 151 has fingers 1511 to 151n. A searcher (not shown) detects multipath and inputs despread-start timing data and delay-time adjustment data of each path to the fingers 1511 to 151n. Each of the fingers 1511 to 151n, has a despread unit for a control channel DPCCH for subjecting the receive signal to despread processing using a code identical with the spreading code of the control channel, integrating the result of despreading, subsequently subjecting the resulting signal to delay processing that conforms to the path and outputting a control-data signal; and a despread unit for a data channel DPDCH for subjecting the receive signal to despread processing using a code identical with the spreading code of the data channel, integrating the result of despreading, subsequently subjecting the resulting signal to delay processing that conforms to the path and outputting a user data signal.
A channel-estimation/AFC circuit 151b estimates the fading characteristic of the communication path using the pilot signal contained in the despread control-data signal output from a selector 151g, executes channel estimation in order to compensate for the effects of fading, and outputs a channel estimation signal. Channel compensation units 151c, 151d multiply the despread control-data signal and despread user-data signal by the complex-conjugate signal of the channel estimation signal to thereby compensate for fading. RAKE combiners 151e, 151f combine the despread signals, from which fading has been eliminated, output from the fingers and output the results to demodulators 152a, 152b, respectively. The demodulators 152a, 152b discriminate “1”, “0” of the user data and control data based upon the signals output from the RAKE combiners 151e, 151f. Since the pilot signal is already known, a selector 153a outputs the control data upon replacing the demodulated pilot signal with the known pilot signal.
The attenuator 153 has multipliers 153b, 153c for multiplying the demodulated user data and control data by a first damping coefficient a that conforms to the degree of reliability, and multipliers 153d, 153e for multiplying the user data and control data by a second damping coefficient β that conforms to the degree of reliability, thereby applying damping. The damping coefficients α, β are set in advance based upon transmission power, the interference environment, etc., by way of example.
The symbol-replica interface 156 multiplies the output signals of the multipliers 153b, 153c by the channel estimation signal (complex signal) that is output from the channel-estimation/AFC circuit 151b, thereby adding on the fading characteristic of the transmission path, and sends the results of multiplication to the corresponding one of the receive demodulators 102a to 102k (see FIG. 17) as symbol-replica signals.
Multipliers 154a, 154b of each of the fingers 1541 to 154n of the re-spreader 154 multiply the user data and control data output from the attenuator 153 by the channel estimation signal (complex signal), thereby adding on the fading characteristic of the transmission path. Re-spread units 154c, 154d spread the user data and control data, onto which fading has been added, by a code identical with the despreading code of the despreader 151, and outputs the spread signals. An adder 154e combines the spread signals, which are output from the respective fingers, by data channel and by control channel, thereby generating interference replicas that are input to the receive demodulators 102a to 102k of the succeeding stage.
The despread-information extraction unit 155 identifies the spreading factor on the transmit side by collecting TFCI bits, which are contained in the control data, over the duration of one frame and inputs the spreading factor to the despreading unit 151a. The latter decides the spreading code of the data channel based upon the spreading factor and performs despreading using this spreading code.
As shown in FIG. 19, the spreading code on the transmit side is the result of multiplying the user identification code (scramble code) SCi, which is for identifying the user, by a channel identification code CCi, which is for identifying the channel (data channel or control channel). The user identification code SCi is not dependent upon the spreading factor SF but the channel identification code CCi of the data channel is dependent upon the spreading factor SF and varies depending upon the spreading factor. It should be noted that the channel identification code of the control channel is fixed because the spreading factor is fixed. FIG. 20A shows a code tree useful in describing the relationship between spreading factor and channel identification code of the data channel, and FIG. 20B is a diagram useful in describing the relationship between channel identification codes of data channels for which SF=2n and SF=2n+1 hold. Here a 1 in the brackets signifies “0” and a −1 signifies “1”. Further, a channel identification code is expressed by Cch,SF,k, where the suffix SF indicates the spreading factor and the suffix k the code number. If SF=4 holds, then four 4-bit channel identification codes Cch,4,0 to Cch,4,3 exist on the basis of FIG. 20A; if SF=8 holds, then eight 8-bit channel identification codes Cch,8,0 to Cch,8,7 exist. Channel identification codes similarly exist for other spreading factors.
In a case where there is only one data channel, the channel identification code of the data channel is Cch,SF,k (where k=SF/4 holds). If the spreading factor SF is equal to 4, therefore, then the channel identification code of the data channel will be Cch,4,1 (1, 1, −1, −1); if SF=8 holds, the channel identification code of the data channel will be Cch,8,2 (1, 1, −1, −1, 1, 1, −1, −1); if SF=16 holds, the channel identification code of the data channel will be Cch,16,4 (1, 1, −1, −1, 1, 1, −1, −1, 1, 1, −1, −1, 1, 1, −1, −1). Thus, the channel identification code of the data channel varies depending upon the spreading factor SF. FIGS. 21(a) and (b) are diagrams useful in describing the relationship between data and channel identification code in a case where the spreading factor SF is equal to 16 [FIG. 21(a)] and a case where the spreading factor SF is equal to 4 [FIG. 21(b)].
Thus, since the spreading-factor information is included in the TFCI bits, the despread-information extraction unit 155 (FIG. 18) collects the TFCI bits over the duration of one frame to identify the spreading factor on the transmit side.
The transmission rate of the control channel is fixed at 15 kbps and the spreading factor of the control channel is fixed. The channel identification code of the control channel therefore is fixed, as mentioned above. For example, the channel identification code of the control channel is fixed and is Cch,246,0. If the user identification code is decided at the time of a call, therefore, the spreading code of the control channel will be evident. However, the transmission rate of the data channel is variable, the spreading factor varies in dependence upon the transmission rate and the channel identification code varies, as mentioned above. Consequently, even though the user identification code is decided at the time of a call, the spreading code of the data channel is unknown until the spreading factor is ascertained.
Accordingly, first the despreading unit 151a despreads only the control channel, finds the spreading factor from the TFCI bits to decide the spreading code of the data channel and then starts despreading the data channel.
FIG. 22 is a diagram showing the structure of each of the receive demodulators 102a to 102k. Each of these demodulators includes fingers 1211 to 121n, RAKE combiners 122a, 122b of the data and control channels, respectively, decoders 123a, 123b of the data and control channels, respectively, and a despread-information extraction unit 124 for identifying and outputting the spreading factor on the transmit side. Each of the fingers 1211 to 121n has a despreader 131 for outputting a despread signal of a signal (interference canceller output), which has been obtained by eliminating an interference signal from the receive signal, in sync with path timing that enters from a searcher (not shown). More specifically, the despreader of the control channel subjects the output signal of the interference canceller to despread processing using a code identical with the spreading code for the control channel, integrates the result of despreading, subsequently subjects the resulting signal to delay processing that conforms to the path and outputs the processed signal. Further, the despreader of the data channel subjects the output signal of the interference canceller to despread processing using a code identical with the spreading code for the data channel, integrates the result of despreading, subsequently subjects the resulting signal to delay processing that conforms to the path and outputs the processed signal.
Combiners 132a, 132b generate transmit signals on the transmit side by adding the symbol replicas of the data and control channels DPDCH, DPCCH to the despread signals of the data and control channels, respectively. A channel-estimation/AFC circuit 133 estimates the fading characteristic of the communication path based upon a pilot signal that enters from a selector 135, and channel correction units 134a, 134b apply channel correction processing to the signals output from the combiners 132a, 132b, respectively, using the respective channel estimation signals, thereby eliminating fading. The RAKE combiners 122a, 122b combine the data signals and control signals, respectively, output from the respective fingers and from which fading has been eliminated, and input the combined signals to the decoders 123a, 123b, respectively. The decoders 123a, 123b apply error correction processing to the user-data signal and control-data signal output from the RAKE combiners 122a, 122b, decode the user data and control data that prevailed prior to encoding and output the decoded data.
The despread-information extraction unit 124 identifies the spreading factor on the transmit side by collecting TFCI bits, which are contained in the control data, over the duration of one frame and inputs the spreading factor to the despreading unit 131. The latter decides the spreading code of the data channel based upon the spreading factor. Furthermore, the despreading unit 131 first despreads only the control channel, finds the spreading factor from the TFCI bits to decide the spreading code of the data channel and then starts despreading the data channel.
Thus, in accordance with an interference canceller, interference can be suppressed by generating the replica of an interference signal and subtracting the replica from the receive signal. This makes it possible to enlarge channel capacity or to reduce transmission power.
The conventional interference canceller is such that if notification of the spreading factor is given in advance, delay can be made several symbols or less. However, in a communication environment in which the amount of data, i.e., the data speed, changes from time to time, as in the case of packet communication, the spreading factor varies from frame to frame or from slot to slot. In such case the spreading factor of the data channel will not be clarified and the channel identification code will not be determined unless the control data (TFCI bits) in one frame or one slot is demodulated. In other words, the conventional interference canceller cannot execute despread processing with regard to the data channel until the channel identification code is clarified. This means that a processing delay in frame or slot units occurs. Consequently, when transmission power control is carried out using TPC (Transmission Power Control), there tends to be an increase in control loop delay and the capacity characteristic deteriorates.
FIG. 23 shows an example of the general construction of the despreading unit 151a in a case where the spreading factor of the data channel DPDCH cannot be identified unless the control information on the control channel DPCCH is demodulated. The despreading unit 151a includes a multiplier circuit 151a-1 for spreading the receive signal by a spreading code CDPCCH for the control channel, a multiplier circuit 151a-2 for spreading the receive signal by a spreading code CDPDCH for the data channel, a delay circuit 151a-3 for delaying the receive signal, and a spreading-code generator 151a-4 for generating the spreading codes CDPCCH, CDPDCH for the control and data channels, respectively.
In the case of the data channel DPDCH, it is necessary to delay the receive signal until the spreading factor is clarified by the TFCI bit and the spreading code CDPDCH for the data channel is determined. The delay circuit 151a-3 is provided for this purpose. Since this delay is a frame or slot delay (the delay differs depending upon the units in which the control information is multiplexed), the delay has a major effect upon TPC (Transmission Power Control).
FIG. 24 is a time chart associated with the block diagram shown in FIG. 23. Here SFi signifies the spreading factor. The spreading factor of the control channel DPCCH is SF1 and is fixed, whereas the spreading factor of the data channel DPDCH takes on various values. In FIG. 24, the control information is multiplexed in frame units. In the interference canceller, the receive signal is delayed by one frame or more until the spreading factor SF is identified and despreading of the data channel is begun. Thereafter, in order to produce an interference replica, a large delay of one frame or more occurs to start interference removal.